Sulfur dioxide determination



Aug. 16, 1960 J. K. CLAUSS SULFUR DIOXIDE DETERMINATION Original FiledJune 20, 1949 2 Sheets-Sheet 1 FIG.

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INVENTOR. JAMES R. CLAUSS United States atent- SULFUR DIOXIDEDETERMINATION James K. Clauss, Palo Alto, Calif., assignor to AlliedChemical Corporation, a corporation of New York 6 Claims. (Cl. 23-232)This invention relates to a method for determining the content of sulfurdioxide in a gas mixture. It relates more particularly to an improvedmethod for continuously determining and recording the concentration ofsulfur dioxide in exit gases from the contact sulfuric acid process.

In the manufacture of sulfuric acid by the contact process, a mixture ofsulfur dioxide and air is passed into contact with a catalyst wherebythe sulfur dioxide is oxidized to sulfur trioxide, and the sulfurtrioxide is then absorbed in sulfuric acid of suitable concentration.The exit gas from a contact sulfuric acid process may contain, forexample, approximately 0.3% by volume of sulfur dioxide, approximately10% by volume of oxygen, and the remainder nitrogen and water vapor,together with a negligibly small amount of carbon dioxide. The gas alsomay contain a small amount of sulfuric acid mist, either mechanicallycarried from the absorbers or formed by combination of unabsorbed sulfurtrioxide with water vapor in the gas.

It is desirable from the standpoint of efficient operation, that theexit gases from the process contain a minimum amount of sulfur dioxide.It is also desirable, in the interest of avoiding contamination of theatmosphere, that the content of sulfur dioxide in the exit gases be low.It is therefore important that the concentration of sulfur dioxide inthe exit gases be readily determined.

It is known to measure the concentration of sulfur dioxide in a gasmixture by aspirating a sample of the gas mixture through a solutioncontaining a measured amount of standard iodine solution and a starchindicator until the solution is decolorized. This test is relativelysimple but it gives only spot information and fails to indicate how theprocess is functioning between tests. Further, it requires the attentionof an operator to carry out the test.

It is also known to determine and record continuously the concentrationof sulfur dioxide in a gas mixture by absorbing the sulfur dioxide in ameasured quantity of electrolyte of known electrical conductivity, suchas an aqueous solution of hydrogen peroxide of known concentration, andmeasuring the change in electrical conductivity of the solution producedby absorption of the sulfur dioxide.

The known processes and devices for measuring the concentration ofsulfur dioxide in a gas mixture by absorption require completeabsorption of the sulfur dioxide present in the sample, and closecontrol of the composition of the absorbent. As a consequence, rate ofgas flow, rate of liquid flow, and temperature and pressure condi tionsare required to be accurately controlled in order to obtain reliableresults.

A principal object of the present invention is to provide an improvedmethod for measuring the concentration of sulfur dioxide in a gasmixture in a more rapid and direct manner than heretofore.

' Other objects of this invention are to provide a process wistsPatented Aug. 16, 1960 for automatically determining and recording theconcentration of sulfur dioxide in a flowing gas mixture which operateson an absorption principle but does not require exact proportioning ofgas and liquid flow rates, nor complete absorption of the sulfur dioxidein the liquid, and which provides a greatly simplified instrument design.

A further object of the present invention is to provide a process forautomatically and continuously determining and recording theconcentration of sulfur dioxide in tail gases from the contact sulfuricacid process which operates on an absorption principle while permittingcomparatively wide variations in the rate of gas flow, rate of liquidflow, and which does not require complete absorption of the sulfurdioxide.

Another object of the present invention is to provide a method formeasuring the concentration of sulfur dioxide in a flowing gas mixturecontaining sulfur dioxide in varying concentration which is continuouslyresponsive to variations in the concentration of sulfur dioxide.

Other objects will in part be obvious and Will in part appearhereinafter.

According to the present invention, the concentration of sulfur dioxidein a gas mixture and especially the concentration of sulfur dioxide intail gases from the contact sulfuric acid process is measured in termsof the electrical conductivity of a solution of sulfur dioxide in waterin equilibrium with the gas mixture, by passing the gas mixture inintimate contact with an aqueous solution of sulfur dioxide in water ofknown electrical conductivity while maintaining a substantially constanttemperature and pressure, controlling the rate of passage of the gasmixture and the relative amounts and the time of contact of the gasmixture and aqueous solution of sulfur dioxide to establish substantialequilibrium between the resulting aqueous solution of sulfur dioxide andsaid gas mixture with respect to sulfur dioxide, separating theresulting aqueous solution of sulfur dioxide from the gas mixture, andmeasuring the electrical conductivity of the resulting separated aqueoussolution of sulfur dioxide while substantially at said temperature andpressure, as an index of the concentration of sulfur dioxide in the gasmixture. Preferably the rate of passage of the gas mixture and therelative amounts of the gas mixture and the aqueous solution of sulfurdioxide are controlled to leave a substantial portion of the sulfurdioxide in the gas mixture undissolved; and contact between the gasmixture and the aqueous solution of sulfur dioxide is maintained forsufficient time to establish substantial equilibrium between theresulting aqueous solution of sulfur dioxide and said gas mixture withrespect to sulfur dioxide. I have discovered that the concentration ofsulfur dioxide in a gas mixture, and especially in tail gases from thecontact sulfuric acid process, can be measured by establishing acondition of equilibrium between the gas mixture and a solution ofsulfur dioxide in distilled water under standardized temperature andpressure conditions, and measuring the electrical conductivity of theresulting solution Since, in accordance with Henrys law, theconcentration of sulfur dioxide in the resulting solution bears aconstant relationship to the partial pressure of sulfur dioxide in thegas mixture in equilibrium with said solution, the electricalconductivity of the solution can be employed as an index of theconcentration of the sulfur dioxide in the gas mixture. Hence, by propercalibration of the conductivity measuring instrument, the concentrationof sulfur dioxide can be determined directly and constantly recorded.

I have further discovered that the oxidizing effect of oxygen in thetail gases from the contact sulfuric acid process can be renderednegligible by rapidly bringing the tail gases to equilibrium with theaqueous solution of sulfur dioxide and that this result can beaccomplished by scrubbing a relatively large amount of the tail gaseswith a relatively small amount of distilled water. It is a feature ofthe present invention that only a portion of the sulfur dioxide presentin the gas mixture is absorbed by the Water. Contrary to prior processesand apparatus which required adsorption of the total amount of sulfurdioxide present in a measured sample of the gas mixture to be analyzed,the present invention merely requires establishing of equilibriumconditions between a solution of sulfur dioxide in water and the gasmixture. By employing a relatively small amount of water it is onlynecessary to dissolve a small amount of sulfur dioxide to establishequilibrium conditions.

I have further discovered that if sulfuric acid mist is present in thetail gases, it can be removed by physical means (such as, trapping it bychange in the velocity of the gas and separating out the precipitatedliquid) without adversely affecting the determination of sulfur dioxide,inasmuch as the solubility of sulfur dioxide in sulfuric acid is low.Thus, by removing residual sulfuric acid from the tail gases withoutsubstantially removing other constituents of the gas mixture (byphysical or chemical means), the determination of the electricalconductivity of the aqueous solution resulting from scrubbing the tailgases with a relatively small amount of distilled water under conditionsof equilibrium provides a direct index of the concentration of sulfurdioxide in the tail gases.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, all aswill be exemplified in the following detailed disclosure and illustratedin the accompanying drawings. The scope of the invention will beindicated in the claims.

In the practice of the process of the present invention in accordancewith a preferred method of procedure, a portion of the tail gases fromthe contact sulfuric acid process is drawn through a sulfuric acid trapwherein sulfuric acid carried by the gases, as entrainment or reactionproduct of residual sulfur trioxide and water vapor, is removed withoutsubstantially altering the concentratiou of sulfur dioxide in the gases,the resulting gases are rapidly brought to a condition of substantialequilibrium with a relatively small amount of a solution of sulfurdioxide in distilled water (as by scrubbing the gases with a relativelysmall amount of distilled water, under standardized substantiallyconstant conditions of temperature and pressure, while maintainingcontact for a sufiicient time to establish substantial equilibrium withrespect to sulfur dioxide between the resulting aqueous solution ofsulfur dioxide and the gases), and the electrical conductivity of thesolution of sulfur dioxide is measured while substantially at saidtemperature and pressure, as an index of the concentration of sulfurdioxide in the tail gases.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings in which Figure 1 is adiagrammatic representation of a complete assembly of a preferred formof apparatus for carrying out the process of the present invention,which comprises a scrubber for passing a relatively large volume of gasin intimate contact with a relatively small volume of water; means forintroducing water into the scrubber; a gas duct for introducing gas intothe scrubber; a sulfuric acid trap connected to the inlet of the gasduct for removing sulfuric acid from the gas prior to its entry into thescrubber; an electrical conductivity cell connected to the scrubber andhaving electrodes immersed in the aqueous solution formed in thescrubber; a thermostatically controlled temperature-regulating jacketsurrounding the scrubber, the conductivity cell and the gas duct formaintaining a constant temperature therein; a liquid duct for removingthe solution from the conductivity cell; a liquid level controlconnected to the liquid duct for maintaining the electrodes of theconductivity cell covered by said solution, and electrical meansconnected to the electrodes for measuring the conductivity of thesolution in the conductivity cell.

Figure 2 is an enlarged elevation, partly in section, of the scrubberand conductivity cell, and

Figure 3 is a sectional elevation of the sulfuric acid trap.

Referring to the drawings, the various parts making up the completeassembly of apparatus shown in Figure 1 are a scrubber 1 for bringingthe gas mixture to be tested into intimate contact with water; aconductivity cell 2, connected to the scrubber and having platinizedelectrodes 23 and 24 which, with their associated measuring andrecording instrument 3, measure and record the electrical conductivityof the aqueous solution of sulfur dioxide formed in the scrubber; aconstantlevel device 4 connected to the scrubber by a tube 5, forsupplying water to the scrubber; a gas duct 6 and its associated flowmeter 7 and sulfuric acid trap 8, for introducing into the scrubber aportion of the exit gases from the sulfuric acid contact process whoseconcentration of sulfur dioxide is to be determined, and an exhaustconduit 11 for withdrawing gases from the scrubber. The scrubber 1,conductivity cell 2 and gas duct 6 are surrounded by atemperatureregulating jacket 9 which is electrically heated by athermostatically controlled electric heater 10. The liquid outlet of theconductivity cell 2 is connected through a flexible tube 51 with aninverted U-tube 52 whose height is adjustable so as to vary the liquidlevel BB in the conductivity cell 2.

The scrubber 1, as shown more clearly in Figure 2, is formed of a 20 mm.Pyrex glass tube 13 having a gas inlet 14 near the bottom thereof andthe gas outlet 11 near the top thereof. The tube 13 is filled to a depthof about 65 mm. with 4 mm. glass beads 16 supported near the lower end15 of the tube by larger beads 17 resting on Vigreaux indentations 18 inthe Wall of the tube 13.

The conductivity cell 2 is formed of a 10 mm. Pyrex glass tube 21 about20 cm. long, connected at its upper end 22 to the lower end 15 of thescrubber 1. Two platinum electrodes 23 and 24, each about 16 sq. mm. inarea, are mounted in the tube 21, about 6 cm. from its upper end 22 andspaced about 6 mm. apart, on platinum wires 25 and 26 which pass throughthe wall of the tube 21 through sealed joints. The portion of tube 21carrying the electrodes is enclosed in a glass envelope 27, forelectrically insulating the wires 25 and 26 from the surroundings, andthe tube 21 and envelope 27 are mounted in a sleeve 28, formed ofBakelite or other insulating material, in which are mounted bindingposts 29 and 30 to which the wires 25 and 26, which pass through thewall of the envelope 27 through sealed joints, are respectivelyconnected. The binding posts 29 and 30 are connected through leads 31and 32 with an electrical measuring and recording instrument 3, which inturn is connected by loads 33 and 34 to a source of 110-volt 60-cyclealternating current.

The measuring and recording instrument 3 may be any suitable instrumentor combination of instruments for measuring and recording theconductance of the solution passing between the electrodes 23 and 24.Thus, it may be an ordinary Wheatstone bridge having the usual pairs ofresistance arms, one arm consisting of the circuit formed by the leads31 and 32, wires 25 and 26 and the aqueous solution of sulfur dioxidebetween the electrodes 23 and 24 whose conductivity is to be determined,and an alternating current galvanometer for measuring the degree ofunbalance in the bridge circuit and thereby changes in resistance(conductance) of the sulfur dioxide solution between the electrodes 23and 24. Instruments of this type, which include means for instantaneousand con- :3 stone bridge arm, and the operation of such instruments arewell known in the art. One type is available in industry as Micromaxrecorders.

The scrubber 1 and conductivity cell 2 are jointly mounted in a tube 91of large diameter having upper and lower closures 92 and 93, thescrubber 1 and gas duct 6 passing through and being supported by theupper closure 92, and the conductivity cell 2 passing through and beingsupported by the lower closure 93. The space within the tube 91surrounding the scrubber 1 and conductivity cell 2 is filled withthermally conductive packing material 94 (such as, aluminum granules,carbon, or the like) thereby providing the temperature-regulating jacket9 for the scrubber 1, conductivity cell 2 and gas duct 6 whereby theyare maintained at a substantially uniform temperature. The tube 91 issurrounded by an electrical heating jacket 10 which is connected byleads 101 and 102 to a relay 103, which in turn is connected by leads104 and 105 with the source of alternating current. The relay iscontrolled by a thermostat 106 which projects through the closure 93into the bed of thermally conductive material 94 in thetemperatureregulating jacket 9 and which is connected to the relay byleads 108 and 109. The temperature-regulating jacket 9 is also providedwith a thermometer 110 for indicating the temperature in the jacket.

. The lower, exit end of the tube 21 is connected by rubber tubing 51with an inverted U-tube 52 for discharging liquid from the conductivitycell 2 into a receiver 53, by means of which the rate of liquid flowingthrough the apparatus may be observed. The inverted U-tube 52 isadjustably mounted so as to permit vertical adjustment of the liquidlevel B-B of the liquid in the conductivity cell 2.

The constant level device for supplying water to the scrubber consistsof a closed vessel 40 connected by a syphon tube 42 and air tube 43 witha water reservoir 41 which is closed by a removable cover 44. The tubes42 and 43 are spaced sufficiently apart in reservoir 41 to prevent airbubbles from entering tube 42. The vessel 40 is also connected by an airtube 45 with a closed air chamber 46 which is connected to theatmosphere through an air inlet tube 47. The vessel 46 is partly filledwith aqueous sodium hydroxide solution, which acts as a liquid seal forthe inner end of tube 47 and serves to remove sulfur dioxide from theair entering the apparatus through tube 47. The tubes 42, 43, 45 and 47are so constructed and arranged as to maintain a substantially constantliquid level in the vessel 40. Thus tubes 42 and 43 terminate in thereservoir 41 near the bottom thereof. Tube 3, which supplies air to thereservoir 41 to replace water removed therefrom, terminates above theliquid level in vessel 40. Tube 42, which supplies water to vessel 40 bya syphon action, terminates considerably below the liquid level invessel 40 so as to maintain a liquid seal for tube 42. The lower end 48of tube 45, which acts as the control of the liquid level in vessel 40,terminates in vessel 40 at the desired liquid level AA and terminates invessel 46 within the air space, considerably above the level of sodiumhydroxide solution in vessel 46. The outlet 49 of the vessel 40 isconnected to tube 5, which is preferably formed of capillary tubing andprovided with a valve 50 for controlling the fiow of water into thescrubber 1.

In operation, the tube 45 serves as a valve for controlling the liquidlevel in vessel 40; when the level AA drops below the end 48 of the tube45 in vessel 40, air is admitted into reservoir 41 through tube 43, tube45, and tube 47 (through the sodiunrhydroxide seal in vessel 46),thereby permitting water to syphon into vessel 40 through tube 42 fromthe reservoir 41. When the level of liquid in vessel 40 risessufficiently to cover the end 48 of tube 45, the flow of air into thereservoir 41 is shut off, thereby shutting oil the flow of water throughthe syphon tube 42.

Sulfuric acid trap 8, as shown more clearly in Figure 3 of the drawings,consists of a chamber 81 having a gas inlet 82 and a gas outlet 83 inwhich is mounted a perforated conical bafile plate 84 with its apexdownward. The inlet 82 of the trap 8 is connected to a T-coupling 86,one arm of which is connected to a pipe 87 which leads to the source ofgases to be measured, shown in the drawings as the gas flue 12. Theother arm of the T-coupling 86 is closed by a removable cleanout plug88. The pipe 87 is slightly inclined downward so as to return any liquidsulfuric acid to the flue 12. The trap 8, conical baffle 84 andconnections are preferably fabricated of lead to increase the efficiencyof the trap by supplementing the physical effect of the change invelocity produced in the trap by the chemical action of the lead on thesmall amount of sulfuric acid which may be present. As required, leadsulfate which accu mulates in T-connection 86 is removed by opening plug88.

The flow meter 7, which is connected to the gas duct 6 by a tube 61 andto the sulfuric acid trap 8 by a tube 62, may be of any usualconstruction, such as a bypass U-tube containing an inert liquid andcalibrated to show the volume of gas flowing through the apparatus.Inasmuch as the exit gases from the sulfuric acid process are atsubstantially atmospheric pressure at the point usually desired fortesting, suction means (not shown) are connected to the exhaust conduit11 for the purpose of drawing the gases to be tested through theapparatus, the gas fiow through the apparatus being controlled by avalve 20 in the conduit 11. 1

In preparing the apparatus for operation, the platinum electrodes of theconductivity cell 2 are cleaned with warm chromic acid and thenplatinized by closing off tube 51 and filling the cell to a point abovethe electrodes 23 and 24 with a 3% aqueous solution of chloro-platinicacid containing 0.03 of lead acetate, connecting the binding posts 29and 30 to a source of 3-volt direct current, and electrolyzing for fourto five minutes, reversing the current every thirty seconds. A blackplatinum deposit is formed on the electrodes. As a precaution, occludedgases and liquid in the coating may be removed, after emptying the celland rinsing with distilled water, by electrolyzing dilute sulfuric acidsolution in the cell for ten minutes, reversing the current everyminute. After rinsing with distilled water, the cell is connected to thescrubber 1 by a suitable clamp, not shown, and the apparatus isassembled as shown in Figure 1.

The apparatus is then calibrated in the following manner: The cover 44of the reservoir 41 is removed, the reservoir is substantially filledwith distilled water or other water suitable for conductivitymeasurement (.preferably, water of known or standardized substantiallyconstant electrical conductivity), and the cover 44 is closed. Vessel 46is filled with 1 Normal sodium hydroxide solution to a depth of severalinches. Tube 42 is filled with distilled water from the reservoir 41 soas to form a syphon discharging into vessel 40. Preferably, theeffective head should not exceed six inches, for a scrubber andconductivity cell of the dimensions given above.

The valve 20 in the gas outlet 11 of the scrubber is adjusted until theflow meter 7 indicates a gas flow of about 10 cubic feet per hourthrough the apparatus. Valve 50 is then opened and water is allowed toflow downward through the scrubber and fill the conductivity cell 2 andthe lower part of scrubber 1. Tube 5-2 is lowered until the liquid flowsfrom it; then it is raised to a point above the level of the electrodes23 and 24, and the flow of water is adjusted by valve 50 in tube 5 untiltube 52 remains filled with water to the height of the bend 54 againstthe effect of the suction in the apparatus. Tube 52 is then graduallylowered while observing the conductivity indicated by the pointer 37 ofthe instrument 3, When the pointer 37 suddenly shifts to the end of thescale registering zero conductivity, thereby indicating that the levelof liquid in the conductivity cell 2 has fallen below the electrodes 23and 2,4, tube 52 is then raised one inch and clamped in that position.This establishes the level .B-B of the solution in the conductivity cell2 at a sufficient height above the electrodes 23 and 24 to avoid theirbecoming uncovered if the pressure in the scrubber should increaseslightly during operation, and yet reduces to a practical minimum theamount of solution to be displaced in the zone of conductivitymeasurement between electrodes 23 and 24, thus reducing the timeinterval for response of the apparatus to variations in the sulfurdioxide concentration of the gases undergoing test. The flow of waterinto the apparatus is then adjusted by ad justment of the valve 50,until the rate of flow of solution into receiver 53 is about 5 ml. perminute, for apparatus having the dimensions referred to above.

The thermostat 106 and relay control 103 are adjusted to maintain atemperature in the temperature-regulating jacket 9 of 40 C., as shown bythe thermometer 110.

Air-sulfur dioxide mixtures, containing known concentrations of sulfurdioxide within the general range of the gases to be tested, are thenpassed through the apparatus, and the readings of the pointer 37 of themeasuring and recording instrument 3 are calibrated in terms of theconcentration of sulfur dioxide in the known gas mixtures.

The apparatus is then installed in a suitable location in the plant andconnected to the test point in the flue 12 by a suitable connection tothe inlet 87.

An apparatus of the type above described and illustrated in the drawingswas employed for the determination of the sulfur dioxide content in theflue gases from the manufacture of sulfuric acid by the contact process,having a composition similar to that referred to above. A stream of thegases at a rate of cu. ft. per hour was passed through the apparatusmaintained at a temperature of 40 C., and the electrical conductivity ofthe solution of sulfur dioxide in distilled water formed in the scrubber1 was continuously measured and recorded as an index of the sulfurdioxide content of the gases for a period of months. As evidenced bychecks against spot analyses of the gases by the Reich test, referred toabove, the maximum error in concentration of sulfur dioxide shown by theapparatus over this long period of continuous operation was only 1% ofthe total sulfur dioxide present in the gases.

It will be evident to those skilled in the art that the invention is notlimited to the foregoing detailed description and that changes can bemade without departing from the scope of the invention.

Thus, various forms of apparatus embodying the principles of the presentinvention can be employed. For example, other forms of scrubbers, inwhich a relatively small volume of liquid is intimately contacted with arelatively large volume of gas so as to attain rapidly conditions ofequilibrium, can be substituted for the scrubber 1. While a constantliquid feed device is desirable for securing automatic operation withminimum supervision, it is not essential that a constant level device beemployed in order to obtain measurement of the sulfur dioxide content ofgas mixtures. Further, other forms of constant level devices may beemployed.

The sulfuric acid trap also may be replaced by other means for removingsulfuric acid without substantial absorption of sulfur dioxide from thegases, and preferably by a change in the velocity of the gas mixture.Further, in the measurement of sulfur dioxide in gas mixtures which aresubstantially free from sulfuric acid, the sulfuric acid trap may beeliminated.

It is not essential that the conditions above described be maintainedfor efiicient operation of the process and apparatus. Thus,substantially identical results are obtained throughout the followingranges of conditions:

Rate of gas flow cu. ft. per hour 5 to 10 Rate of water flow ml. perhour 150 to 450 Lenth of glass bead packing in scrubber, ft 1 to 5 Thus,the process and apparatus of the present invention are not restricted tothe exact proportioning of gas to liquid required by previously knownprocesses and apparatus. It is suflicient that merely an amount of gasbe contacted with the stream of water to provide an excess over theequilibrium concentration of sulfur dioxide in water at the temperatureand pressure of operation and that contact be maintained for asufficient time to establish substantial equilibrium. Further, since theinvention depends upon the establishment of equilibrium conditions andnot upon complete absorption of the sulfur dioxide from a sample of thegas to be tested, variations in the gas and liquid flow have no adverseeffect upon the accuracy of the determinations. However, sincestandardization and control of conditions within practical limitssimplify continuous operation without close supervision, they arepreferred.

For efiicient operation, it is desirable that the temperature andpressure be maintained substantially constant, otherwise the accuracy ofthe device will be adversely affected. The specific embodiment of theinvention described above employs a temperature of 40 C. andsubstantially atmospheric pressure. The invention is not limited to theuse 'of such temperature and pressure conditions, however, and othercombinations of temperatures and pressure can be employed by calibratingthe apparatus for such temperature and pressure. Calibration of theapparatus above described at various temperatures from room temperature(about 20 C.) to 45 C. also gave substantially identical results otherthan a difference in the calibration curve. For ease in operation,temperatures below room temperature are less desirable since theyrequire cooling, and temperatures above 45 C. are less desirable owingto the decreased solubility of sulfur dioxide in water.

It is a feature of the present invention that the volume of waterpassing downward through the scrubber 1 is small as compared with thevolume of gas in contact therewith at any time in the scrubber. As aconsequence, the scrubber 1 is in effect an extended film of water oflarge surface area in contact with the gases, with the result thatequilibrium conditions between the gas and the liquid are rapidlyestablished at the above rates of gas and liquid flow. Since gas entersthe scrubber near the bottom thereof, and passes countercurrent to thedownwardly flowing solution of sulfur dioxide in water formed in thescrubber, the solution passing from the scrubber into the conductivitycell is substantially in equilibrium with the gas mixture entering thescrubber; and since the electrodes 23 and 24 are but a short distancebelow this point, the conductivity of the solution of sulfur dioxide ismeasured within a short time interval after contact with the enteringgases. Hence, the measurement of the conductivity of the sulfur dioxidesolution in the conductivity cell 2 provides an accurate measure of theconcentration of sulfur dioxide in the gases entering the scrubber.

It is a further feature of the present invention that although sulfurousacid is oxidized to sulfuric acid by oxygen, and the presence ofsulfuric acid in the sulfurous acid formed by absorption of sulfurdioxide in water markedly influences the conductivity of the sulfurousacid solution, the amount of sulfuric acid formed in accordance with theprocess of the present invention is insufiicient to substantially affectthe measurement of sulfur dioxide content obtained in accordance withthe present invention, notwithstanding the substantial presence ofoxygen in the tail gases.

It is an additional feature of the present invention that the presenceof carbon dioxide in sulfur dioxide-containing gases has little effectupon the results obtained, owing to the fact that the ionizationconstant of carbonic acid is much smaller than that of sulfurous acid.

exit gases from the contact sulfuric acid process, it is,

not limited thereto but can be employed for the determination andrecording of sulfur dioxide concentrations in other gas mixtures thatare essentially free from other constituents which dissolve in water toform an electrolyte or which can be readily freed from such constituentswithout substantial reduction in the concentration of sulfur dioxide inthe gas mixture. As employed herein and in the claims, the expressiongas mixtures that are essen tially free from other constituents includesgas mixtures containing constituents which dissolve in Water to form anelectrolyte having a considerably lower conductivity than that ofsulfurous acid, as well as gas mixtures free from other constituentswhich dissolve in water to form an electrolyte.

The process above described is especially adapted for the determinationand recording of sulfur dioxide concentrations in gas mixtures whereinthe concentration of sulfur dioxide ranges from 0.05 to 2.00%. Where alesser degree of accuracy is permissible, the process can be employedfor the measurement of gas mixtures of considerably higher sulfurdioxide concentration, for example, gases containing 10% of sulfurdioxide.

This application is a division of my copending application Serial No.100,269, filed June 20, 1949, now abandoned.

The apparatus described above and shown in the drawings is claimed in mycopending application Serial No. 630,169, filed December 24, 1956.

I claim:

1. A method of determining the concentration of sulfur dioxide in aflowing gas mixture that is essentially free from other constituentswhich dissolve in Water to form an electrolyte which comprises passingthe gas mixture in intimate contact with a solution of sulfur dioxide inwater of known electrical conductivity while maintaining a substantallyconstant temperature and pressure, controlling the rate of passage ofthe gas mixture and the relative amounts of the gas mixture and aqueoussolution of sulfur dioxide to dissolve a portion of the sulfur dioxidein the solution and leave a substantial portion of the sulfur dioxide inthe gas mixture undissolved, maintaining contact of the gas mixture withthe aqueous solution of sulfur dioxide for sufficient time to establishsubstantial equilibrium between the resulting aqueous solution of sulfurdioxide and said gas mixture with respect to sulfur dioxide, separatingthe resulting aqueous solution of sulfur dioxide from the gas mixture,and measuring the electrical conductivity of the resulting separatedaqueous solution of sulfur dioxide while substantially at saidtemperature and pressure, as an index of the concentration of sulfurdioxide in the gas mixture.

2. A method of determining the concentration of sulfur dioxide in a gasmixture that is essentially free from other constituents which dissolvein water to form an electrolyte which comprises dissolving sulfurdioxide in water of constant electrical conductivity to form an aqueoussolution of sulfur dioxide, passing the gas mixture in intimate contactwith said aqueous solution of sulfur dioxide while maintaining astandardized temperature and pressure, controlling the rate of passageof the gas mixture and the relative amounts of gas mixture and aqueoussolution of sulfur dioxide to dissolve a portion of the sulfur dioxidein the solution and leave a substantial portion of the sulfur dioxide inthe gas mixture undissolved, maintaining contact of the gas mixture withthe aqueous solution of sulfur dioxide for sufficient time to establishsubstantial equilibrium between the resulting aqueous solution of sulfurdioxide and said gas mixture with respect to sulfur dioxide, separatingthe resulting aqueous solution of sulfur dioxide from the gas mixture,and measuring the electrical conductivity of the resulting separatedaqueous solution of sulfur dioxide while substantially at saidtemperature and pressure, as an index of the concentration of sulfurdioxide in the gas mixture.

3. A method of determining the concentration of sulfurdioxide in a gasmixture that is essentially free from other constituents which dissolvein water to form an electrolyte, which comprises passing the gas mixturefirst in contact with an aqueous solution of sulfur dioxide in Water ofconstant electrical conductivity, while maintain ing a substantiallyconstant temperature and pressure, and then in contact with water ofconstant electrical conductivity to form said aqueous solution of sulfurdioxide, controlling the rate of passage of the gas mixture and therelative amounts of gas mixture and aqueous solution of sulfur dioxideto leave a portion of the sulfur dioxide in the gas mixture undissolved,maintaining contact of the gas mixture with the aqueous solution ofsulfur dioxide for suflicient time to establish substantial equalibriumbetween the resulting aqueous solution of sulfur dioxide and said gasmixture with respect to sulfur dioxide, separating the resulting aqueoussolution of sulfur dioxide from the gas mixture, and measuring theelectrical con ductivity of the resulting separated aqueous solution ofsulfur dioxide while substantially at said temperature and pressure, asan index of the concentration of sulfur dioxide in the gas mixture.

4. A method of determining the concentration of sulfur dioxide in a gasmixture that is essentially free from other constituents which dissolvein water to form an electrolyte, which comprises passing the gas mixturein contact with an aqueous solution of sulfur dioxide in distilled waterwhile maintaining a substantially constant temperature and pressure,controlling the rate of passage of the gas mixture and the relativeamounts of the gas mixture and aqueous solution of sulfur dioxide toleave at least a substantial portion of the sulfur dioxide in the gasmixture undissolved, maintaining contact of the gas mixture with theaqueous solution of sulfur dioxide for sufiicient time to establishsubstantial equilibrium between the resulting aqueous solution of sulfurdioxide and said gas mixture with respect to sulfur dioxide, separatingthe resulting aqueous solution of sulfur dioxide from the gas mixture,passing the resulting sulfur dioxide-containing gas mixture in contactwith distilled water to form the aqueous solution of sulfur dioxide, andmeasuring the electrical conductivity of the separated aqueous solutionof sulfur dioxide while substantially at said temperature and pressure,as an index of the concentration of sulfur dioxide in the gas mixture.

5. A method of determining the concentration of sulfur dioxide in tailgases from the manufacture of sulfuric acid which comprises removingsulfuric acid from the tail gases without substantially removing sulfurdioxide, flowing a stream of relatively small volume of water ofstandardized substantially constant electrical conductivity through agas and liquid contact zone countercurrent to and in contact with astream of relatively large volume of the gas mixture, so as to form anaqueous solution of sulfur dioxide, withdrawing the resulting aqueoussolution of sulfur dioxide from said contact zone, controlling the ratesof flow of the water and the gas mixture passing through said contactzone so that only a portion of the sulfur dioxide in the gas mixture isdissolved in the water and a portion of the sulfur dioxide is left inthe exit gas mixture, maintaining contact of the gas mixture with theaqueous solution of sulfur dioxide in the contact Zone for sufiicienttime to establish substantial equilibrium between the resulting aqueoussolution of sulfur dioxide and said gas mixture with respect to sulfurdioxide, maintaining the contact zone under standardized constanttemperature and pressure conditions, and measuring the electricalconductivity of the aqueous solution of sulfur dioxide withdrawn fromthe contact zone, while substantially at said temperature and pressure,as an index of the concentration of sulfur dioxide in the gases.

6. A method of determining the concentration of sulfur dioxide in tailgases from the manufacture of sulfuric acid by the contact process whichcomprises '11 removing sulfuric acid from a sample of the tail gases bychange in the velocity of the gases, passing a relatively large volumeof the resulting gases through a scrubber countercurrent to a relativelysmall volume of distilled water while substantially maintaining astandardized constant temperature and pressure, withdrawing theresulting aqueous solution of sulfur dioxide from said scrubber at saidtemperature and pressure, controlling the rates of flow of the water andgases passing through said scrubber so that only a portion of the sulfurdioxide in the gases is dissolved in the water and a portion of thesulfur dioxide is left in the exit gases, maintaining contact of thegases with the resulting aqueous solution of sulfur dioxide in thescrubber for suffi cient time to establish substantial equilibriumbetween the resulting aqueous solution of sulfur dioxide and ReferencesCited in the file of this patent UNITED STATES PATENTS 1,475,000 Cooperet a1. Nov. 20, 1923 OTHER REFERENCES Thomas et a1.: Automatic Apparatusfor Determination of Small Concentrations of Sulfur Dioxide in Air,Ind-and Eng. Chem., Anal. Ed., vol. 18, No. 5, pp. 383-387 (1946).

1. A METHOD OF DETERMINING THE CONCENTRATION OF SULFUR DIOXIDE IN AFLOWING GAS MIXTURE THAT IS ESSENTIALLY FREE FROM OTHER CONSTITUENTSWHICH DESOLVE IN WATER TO FORM AN ELECTROLYTE WHICH COMPRISES PASSINGTHE GAS MIXTURE IN INTIMATE CONTACT WITH A SOLUTION OF SULFUR DIOXIDE INWATER OF KNOWN ELECTRICAL CONDUCTIVITY WHILE MAINTAINING A SUBSTANTIALLYCONSTANT TEMPERATURE AND PRESSURE, CONTROLLING THE RATE OF PASSAGE OFTHE GAS MIXTURE AND THE RELATIVE AMOUNTS OF THE GAS MIXTURE AND AQUEOUSSOLUTION OF SULFUR DIOXIDE TO DISSOLVE A PORTION OF THE SULFUR, DIOXIDEIN THE SOLUTION AND LEAVE A SUBSTANTIAL PORTION OF THE SULFUR DIOXIDE INTHE GAS MIXTURE UNDISSOLVE, MAINTAINING CONTACT OF THE GAS MIXTURE WITHTHE AQUEOUS SOLUTION OF SULFUR DIOXIDE FOR SUFFICIENT TIME TO ESTABLISHSUBSTANTIAL EQUILIBRIUM BETWEEN THE RESULTING AQUEOUS SOLUTION OF SULFURDIOXIDE AND SAID GAS MIXTURE WITH RESPECT TO SULFUR DIOXIDE, SEPARATINGTHE RESULTING AQUEOUS SOLUTION OF SULFUR DIOXIDE FROM THE GAS MIXTURE,AND MEASURING THE ELECTRICAL CONDUCTIVITY OF THE RESULTING SEPARATEDAQUEOUS SOLUTION OF SULFUR DIOXIDE WHILE SUBSTANTIALLY AT SAIDTEMPERATURE AND PRESSURE, AS AN INDEX OF THE CONCENTRATION OF SULFURDIOXIDE IN THE GAS MIXTURE.